[obican]

Oh boy... I hadn't realized you had no idea about this subject.

[Brightcolours]

I guess the answer to my questions is no, you are not clear on the fundamentals.

 

I give up, you are only interested to flame my knowledge and pretend you have knowledge which you do not.

You thinking that front or back focus of a lens has any connection to the focus screen, and that the focus screen should be adjusted to different lenses, it tell tale enough.

That is fine. Have a nice new year!

[obican]

Your knowledge? Boy, you know nothing. You have been saying otherwise all the time but everytime someone asks you anything you try to divert. 

 

To put everything together really short:

 



 

 

This is a DSLR cross section. There are two mirrors, one of them diverts the light to the focusing screen, other to the AF sensor. When an image is being made, both get out of the way and light reaches the image sensor.

 

In order for AF sensor to function correctly, it has to be very carefully calibrated so that the distance the light takes to reach the AF sensor should be exactly the same as it takes after the mirror flips up, reaching the image sensor. If there is a miscalibration within the lens, lens mount, main mirror, secondary mirror, image sensor or AF sensor, you dial in a microadjustment so that the camera compensates for the amount. You can set this value per lens or per camera body on most cameras.

 

The same thing goes for the focusing screen too. If there is a miscalibration anywhere within the system, the focused area on the viewfinder screen will not be the same as the image sensor.

 

Viewfinder screen is (in theory, most of the time it's not really) carefully shimmed to ensure it is calibrated. However, if there is a problem with the lens or something had occured with the mirror later or if the whole system is not up to specs to begin with (quite common actually), you can't rely on the viewfinder to ensure good focus. You can't compensate for this electronically neither.

 

Got it now?

 

It has nothing to do with the PDAF algorythms, it has nothing to do with lens taking a long step. It is just a mismatch of distances because belive it or not, your camera is made quite cheaply.

 

Prove me wrong.

[Brightcolours]

You have been claiming this: with a front/back focussing lens the focus screen has to be shimmed to calibrate the focus screen to that particular lens.

 

That is what you have been claiming, and that is just weird nonsense. If the focus screen is in place, it is fine for every lens you put on the camera.

 

About AF: you show a remarkable limited amount of understanding of how things work. Evidently the AF sensor needs to be in position. Evidently, the mirror needs to not be misaligned. If either of those are not right, the camera has to be repaired as AF will not be ok for any lens. Most cameras sold are totally fine concerning the mirror alignment and the AF sensor placement/calibration.

 

AF issues with certain lenses then come from either of two things: the lens does wrong sized steps at the end of the AF algorithm, which the camera does not expect and does not check (this is for speed reasons). Or the lens has colours not all focussing on the same plane, and the AF sensor catches onto one of those different focussed colours.

 

In the former case one has to send in the lens for calibration.

[obican]

You can't always. Try sending a Samyang for that, for example.

 

Also, if you want people to behave as if you're not dog poo, seriously dial down your tone. You have probably insulted everyone on this forum here and there and believe it or not, probably nobody really likes you. I'm taking my sweet time to beat some sense into that thick skull of yours but only thing I get is shit coming out of your fingers, which would originally come out from your mouth but this is the internet.

 

So, unlike you, I'll say this only once: I'm done with you. Go ahead, post another message and have the last word. Nobody cares you do since nobody really takes you for anything serious. I just hope you don't realize what a useless asshat you have been someday and commit suicide all alone in your apartment and frame me for it.

[toni-a]

Folks this is new year come on let's be peaceful.

I think you are close to an agreement.

We usually align the focus screen to the sensor not to a specific lens,  here's another presentation of the SLR light paths

 



 

a camera will have front back focus with all lenses if AF sensor and image sensor are not aligned, the image is in sharp focus on the AF sensor but not necessarily on the imaging sensor, with alignment the camera will have accurate autofocus, why this shouldn't apply to the focusing screen ?

Thanks   to advances in manufacturing process this is very rare to happen, normally all three are aligned, however when they aren't you can realign them.

[Brightcolours]

You can't always. Try sending a Samyang for that, for example.

 

Also, if you want people to behave as if you're not dog poo, seriously dial down your tone. You have probably insulted everyone on this forum here and there and believe it or not, probably nobody really likes you. I'm taking my sweet time to beat some sense into that thick skull of yours but only thing I get is shit coming out of your fingers, which would originally come out from your mouth but this is the internet.

 

So, unlike you, I'll say this only once: I'm done with you. Go ahead, post another message and have the last word. Nobody cares you do since nobody really takes you for anything serious. I just hope you don't realize what a useless asshat you have been someday and commit suicide all alone in your apartment and frame me for it.

Samyang lenses are not AF. So, they can't front/back focus with AF.

What does the focus screen have to do with front/back focus? Totally nothing at all.

 

That you want to continue to flame just says a lot about you.

[toni-a]

Samyang lenses are not AF. So, they can't front/back focus with AF.

What does the focus screen have to do with front/back focus? Totally nothing at all.


That you want to continue to flame just says a lot about you.

To avoid letting this pointless war go further let me answer instead of Obican, BC I have never used any humiliating words and will never do, so no need to use any bad words on me.

Since it's manual focus you can't obviously do autofocus calibration however if your focus screen is not aligned with your sensor then ALL the lenses you use will have focus issues when using manual focus.

In that case you have to calibrate the focus screen to the camera sensor and all your lenses will then focus accurately using manual focus.

I am ready and open for any scientific discussion about the subject as long as it stays within the internet netiquette.

[wim]

Samyang lenses are not AF. So, they can't front/back focus with AF.

What does the focus screen have to do with front/back focus? Totally nothing at all.

 

That you want to continue to flame just says a lot about you.

 

Technically speaking, there currently are Samyang AF lenses, be it for Sony. Canon and Nikon AF lenses have been announced.

 

Kind regards, Wim

[Arthur Macmillan]

I still like Brightcolours!  And Wim, Toni-a, Obican, Dave's, and the rest.  We are all at some level of knowledge, and trying to push that knowledge a little further.  Sadly, this has degenerated in to a DPReview style discussion.  I thought all of you guys were friends who love there hobby.  I admit I know less than most of the people on the forum.  I do get the impression that the two who know the most are at opposite ends of the politeness spectrum.  Rude behavior, in my experience is often at its worst between friends who respect each others skills, because then it becomes a friendly rivalry.

 

OK, Brightcolours can go a bit far.  That means he loves you, by my way of thinking.

 

He has pointed out valuable information about the focus screen which I personally find fascinating!  I've never considered changing a focus screen because if there is a risk that it will be out of focus, what's the point.

 

@ Brightcolours AKA "asshat" (just kidding!)  Pretend I know nothing about cameras.  Is it really that easy to change focus screens on the 6D, and the secret is to stick with Canon made screens?  That is VERY valuable information, and all of us Canon shooters are adding to our knowledge whether we thank BC or not!  My question:  Does this apply to the Canon 5D Mark II? 

 

I listen to you guy's arguments that demonstrated the superiority of the 6D over the 5D Mark II, but I was given a pristine 5D Mark II, and a 1.4x teleconverter, my Christmas gifts!  So, needless to say, I've gladly accepted it, and will finally experience the world of FF shooting!  I will attempt to look this up on the internet.  I just received the camera yesterday evening.  But, how great it would be if I could switch to the fast screen, with out having to be a camera tech. 

 

@ Wim, and Klaus (who made a reference in a different thread).  You have made comments that are thought provoking.  Being not at the level to really need to know how it's done, but still curious about MFA.  Wim, you mention that the lens is shimmed at the factory if necessary to adjust any back/front focusing inherent in the lens (paraphrasing of course).  Klaus, you mentioned that some lens had a lens defect that (probably) Sigma was able to correct remotely using the electronic usb pedestal (pretty much lying here).  This combines in my little pea brain and makes me wonder.  Is the EUP really just for firmware updates?  what if it can shift the alignment (close and far points) of the focus group.  Then the lens could be produced with less exacting tolerances, and MFA at the lens as a final step.  Or maybe this is already known to be the case...I just like reading about stuff.  Personally, I have hoped that the MFA could be done on the lens side, and be a quick fix, instead of a repair issue.  A self-diagnostic.  

 

Then in my over active imagination I picture the pedestal and camera being linked together.

 

I could tell you my guess of how I think it is done currently in camera, but it would just be a guess, and I don't really need to know yet.  I have too much new gear to play with to even care about MFA, and focus screens.  That will wait until I have nothing new, and have to perfect what I have as the next great improvement.